Bottom Line:
Electrical pulses have been used to enhance uptake of molecules into living cells for decades.In vivo imaging of luciferase activity after electrovaccination demonstrated a rapid onset (minutes) and a long duration (months) of transgene expression.This study provides important insights to how DNA delivery by intradermal electrovaccination affects the local immunological responses of the skin, transgene expression and clearance of the plasmid.

Background: Electrical pulses have been used to enhance uptake of molecules into living cells for decades. This technique, often referred to as electroporation, has become an increasingly popular method to enhance in vivo DNA delivery for both gene therapy applications as well as for delivery of vaccines against both infectious diseases and cancer. In vivo electrovaccination (gene delivery followed by electroporation) is currently being investigated in several clinical trials, including DNA delivery to healthy volunteers. However, the mode of action at molecular level is not yet fully understood.

Methodology/principal findings: This study investigates intradermal DNA electrovaccination in detail and describes the effects on expression of the vaccine antigen, plasmid persistence and the local tissue environment. Gene profiling of the vaccination site showed that the combination of DNA and electroporation induced a significant up-regulation of pro-inflammatory genes. In vivo imaging of luciferase activity after electrovaccination demonstrated a rapid onset (minutes) and a long duration (months) of transgene expression. However, when the more immunogenic prostate specific antigen (PSA) was co-administered, PSA-specific T cells were induced and concurrently the luciferase expression became undetectable. Electroporation did not affect the long-term persistence of the PSA-expressing plasmid.

Conclusions/significance: This study provides important insights to how DNA delivery by intradermal electrovaccination affects the local immunological responses of the skin, transgene expression and clearance of the plasmid. As the described vaccination approach is currently being evaluated in clinical trials, the data provided will be of high significance.

Mentions:
As in vivo monitoring of luciferase expression does not show individual cells and the light emission spreads beyond the location of transfected cells, histochemical staining of β-galactosidase was used to investigate the location of the DNA transfected cells in the skin. This demonstrated that the use of needle array electrodes produced a uniform transfection of the treated volume, resulting in a large area of transfected cells confined to the space between the needle rows (Fig. 2a), with the majority of the transfected cells concentrated around the panniculus carnosus muscle layer in the deepest layer of the skin (Fig. 2c–e). Transfected cells were also found in the hypodermis (Fig. 2c–e) and in sparse numbers through the dermis and epidermis (Fig. 2e, f). However, no expression was observed in actual muscle cells. No background staining was seen in untreated skin (Fig. 2g) and neither luciferase nor LacZ expression was detected in the tissue immediately below the injected skin (data not shown). The data demonstrate that intradermal DNA delivery in combination with electroporation results in cell transfection throughout all layers of the skin and yields faster, higher and more consistent gene expression, compared to intradermal gene delivery alone.

Mentions:
As in vivo monitoring of luciferase expression does not show individual cells and the light emission spreads beyond the location of transfected cells, histochemical staining of β-galactosidase was used to investigate the location of the DNA transfected cells in the skin. This demonstrated that the use of needle array electrodes produced a uniform transfection of the treated volume, resulting in a large area of transfected cells confined to the space between the needle rows (Fig. 2a), with the majority of the transfected cells concentrated around the panniculus carnosus muscle layer in the deepest layer of the skin (Fig. 2c–e). Transfected cells were also found in the hypodermis (Fig. 2c–e) and in sparse numbers through the dermis and epidermis (Fig. 2e, f). However, no expression was observed in actual muscle cells. No background staining was seen in untreated skin (Fig. 2g) and neither luciferase nor LacZ expression was detected in the tissue immediately below the injected skin (data not shown). The data demonstrate that intradermal DNA delivery in combination with electroporation results in cell transfection throughout all layers of the skin and yields faster, higher and more consistent gene expression, compared to intradermal gene delivery alone.

Bottom Line:
Electrical pulses have been used to enhance uptake of molecules into living cells for decades.In vivo imaging of luciferase activity after electrovaccination demonstrated a rapid onset (minutes) and a long duration (months) of transgene expression.This study provides important insights to how DNA delivery by intradermal electrovaccination affects the local immunological responses of the skin, transgene expression and clearance of the plasmid.

Background: Electrical pulses have been used to enhance uptake of molecules into living cells for decades. This technique, often referred to as electroporation, has become an increasingly popular method to enhance in vivo DNA delivery for both gene therapy applications as well as for delivery of vaccines against both infectious diseases and cancer. In vivo electrovaccination (gene delivery followed by electroporation) is currently being investigated in several clinical trials, including DNA delivery to healthy volunteers. However, the mode of action at molecular level is not yet fully understood.

Methodology/principal findings: This study investigates intradermal DNA electrovaccination in detail and describes the effects on expression of the vaccine antigen, plasmid persistence and the local tissue environment. Gene profiling of the vaccination site showed that the combination of DNA and electroporation induced a significant up-regulation of pro-inflammatory genes. In vivo imaging of luciferase activity after electrovaccination demonstrated a rapid onset (minutes) and a long duration (months) of transgene expression. However, when the more immunogenic prostate specific antigen (PSA) was co-administered, PSA-specific T cells were induced and concurrently the luciferase expression became undetectable. Electroporation did not affect the long-term persistence of the PSA-expressing plasmid.

Conclusions/significance: This study provides important insights to how DNA delivery by intradermal electrovaccination affects the local immunological responses of the skin, transgene expression and clearance of the plasmid. As the described vaccination approach is currently being evaluated in clinical trials, the data provided will be of high significance.